For the detection of gamma and X-ray radiation in particular in CT, SPECT and PET systems, inter alia direct-converting detectors are used, or their use is intended, which are based on semiconducting materials such as CdTe, CdZnTe, CdZnTeSe, CdTeSe, CdMnTe, InP, TIBr2, HgI2. However the effect of polarization occurs with these materials, in particular with a high radiation flux density that is necessary for CT devices.
Polarization refers to the reduction in the detected count rate with high photon fluxes or radiation fluxes. This polarization is caused by the very low mobility of charge carriers, predominantly of electron vacancies or holes, and by the concentration of intrinsic vacancies in the semiconductor. In other words, polarization occurs as a result of the reduced electric field caused by stationary charges bound to vacancies, which function as absorption and recombination centers for the charge carriers generated by the radiation. This results in a reduced charge carrier lifetime and mobility, which in turn leads to a reduction in the detected count rate with high photon fluxes.
However a radiation detector has to have a high charge carrier lifetime and mobility so that the electrons and holes that occur during irradiation can be separated. The maximum detectable radiation flux of a direct converter is otherwise limited as a result of polarization. For this reason it has hitherto not been possible to convert high radiation densities, such as those predominantly employed in computed tomography, directly and without loss into electrical pulses.
The publication US 2008/0164418 A1 describes a method for reducing polarization in a semiconductor detector, in which electrical signals are generated in response to the absorption of an ionizing radiation in the semiconductor, wherein a space charge occurs in the semiconductor and, as a function of the space charge generated, the semiconductor is irradiated with one or more wavelengths of IR radiation so as to at least partially reduce the polarization in the semiconductor and thus its effect on the electrical signals. The IR radiation used has a maximum energy of 1.57 eV with a wavelength of at least 790 nm.